The body louse has "the smallest number of detoxification enzymes observed in any insect," researchers write, suggesting new repellents could one day help rid us of the tiny mites that have been our companions for millions of years.

"The ecology of lice is very, very simple. It either lives in your hair or on your clothing, and it has one type of meal, and that's blood," says University of Illinois's Professor Barry Pittendrigh.

"So most of the genes that are responsible for sensing or responding to the environment are very much reduced," says Pittendrigh, who coordinated the research of 28 institutions in the United States, Europe, Australia and South Korea.

Dr Stephen Cameron of CSIRO Entomology examined the gene responsible for how the louse responds to its environment.

"Lice have a simple life either in your hair or on your body with blood as their food. So most of the genes responsible for sensing their environment have been reduced," says Cameron.

"One gene in particular is responsible for normal cell maintenance and in the body louse this gene is missing. This can give valuable insights into diseases and human ageing."

Important vector

Known as Pediculus humanus humanus, the body louse has played its unsanitary part in history, blamed for spreading typhus and trench fever to Napoleon's retreating army in Russia in 1812.

"As an obligatory parasite of humans, the body louse is an important vector for human diseases, including epidemic typhus, relapsing fever, and trench fever," the study says.

"Thus, the body louse genome project offers unique information and tools to use in advancing understanding of coevolution among vectors, symbionts, and pathogens."

Completely dependent on humans for its survival, the louse will die if separated from its host for very long.

In turn a tiny microbe, the bacterium Candidatus Riesia pediculicola, lives inside it and produces pantothenate (vitamin B5), an essential nutrient for its host's survival.

The study also sequenced the genome of the microbe and pointed out that the research was key to understanding the co-evolution of disease-carrying parasites and their bacterial co-conspirators.

"Lice have been used to understand human evolution and migration. They've been used to estimate when we started wearing clothing," says Pittendrigh.

"The genome should also help us develop better methods of controlling both head and body lice."